The Stirling engine
was invented by Rev. Robert Stirling in 1816.He was a Scottish
minister. At that time, Stirling engineswere recognized as
a safe engine that could not explodelike steam engines
of that era often did.

My friend Adolf Cortel
from Spain shows two modelsof Stirling motors
to VIP's at POS2 (Physics on Stage 2002)in Noordwijk (The
Netherlands).
The motor on the leftis of the kind illustrated
in the following sketch.

THE
STIRLING MOTOR WORKING PRINCIPLE:The cycle of a generic
Stirling engine is as follows:Air at the bottom
of the cylinder (E) is heated,thus expanding and
forcing the piston (A) upward.At this time the displacer
(B) is driven downwardto the bottom of the
cylinder. Since the displaceris of a smaller diameter
than the cylinder, thehot air rushes around
the displacer to the coolend of the engine
(F). Once in the top end ofthe cylinder, the
hot air begins to contract,sucking the piston
downward. Now the displacermoves upward, forcing
all the cool air fromthe top end of the
cylinder into the bottom end.Here the air is heated
and the cycle begins again.Note the 90° phase
shift between the displacerand the active piston.
Furthermore, this motor runscompletely silent!

This is the beautiful
Stirling engine from EXERGIA GERMANY(WWW.EXERGIA.DE)
which incredibly runs on unconcentratedsolar radiation on
its top surface. The same firm sellsalso other interesting
engines both in kit and fully assembled.

ANOTHER
INTERESTINGCONFIGURATIONsymilar to that of
the above animation:Owing to the simplified
mechanism of the 90° shifteddisplacements of the
pistons, this structure could beeasier to make.

CONSTRUCTION
TRICKS AND HINTS

1.
In small engines the connecting rod small end fromthe piston or displacer
to the crankshaft may be done by meansof an intermediary
small piece of flexible rubber tubing.2.
In small engines the piston may be in the form of a thin flexiblerubber membrane, like
in those barely visible in the above photographyof Adolf Cortel from
Spain and in the following image:

3.
Long life, self lubricating and very low friction pistons are made ofgrafite, with the
cylinder made of glass tube. Suitable cylindersare made by a glass
syringe.4.
As in the above "cofee cup" model Stirling motor, the bushes and the axlebearings are suitably
made by teflon. Remember to keep frictions as smallas possible.

MY
STIRLING MOTOR

This is a detail of
a Stirling motor constructed in the Laboratoryof
the Museum of Physics of Cagliari University.Cylinder: glass; piston:
graphite; displacer: styrofoam. Undera working temperature
difference of 20 °C it turns at 150 rpm.Note the couplig between
the piston and the connectingrod made by a spring
of inox harmonic wire.